1. Field of the Invention
This invention relates to an image processing method and a computer readable medium for image processing, for rendering an observation object by using volume data.
2. Description of the Related Art
A technique for visualizing the inside of a three-dimensional object has attracted public attention with the advance of image processing technology using a computer in recent years. Particularly in the medical field, medical diagnosis using a CT (Computed Tomography) apparatus or MRI (Magnetic Resonance Imaging) apparatus has been performed widely because a lesion can be detected early by visualizing the inside of a living body.
On the other hand, volume rendering is known as a method for obtaining a three-dimensional image of the inside of an object. In volume rendering, ray is emitted onto a three-dimensional voxel (micro volume element) space to thereby project an image on a projection plane. This operation is referred to as ray casing. In ray casting, a voxel value is acquired from a voxel at each sampling point which is sampled at a regular interval along the path of the ray.
The voxel is a unit for constituting a three-dimensional region of an object. The voxel value is a specific data expressing characteristic such as a density value of the voxel. The whole object is expressed by voxel data which is a three-dimensional arrangement of the voxel value. Generally, two-dimensional tomogram data obtained by CT is collected along a direction perpendicular to each sectional layer, and voxel data which is the three-dimensional arrangement of voxel value is obtained by performing necessary interpolation.
FIGS. 12A and 12B show a situation in which an organ is rendered using volume data. When an organ containing a residual is rendered, if a contrast medium is injected to clean and visualize the residual in an intestine 50 and the inside of the intestine 50 is cleaned, the contrast medium remains in the intestine 50 and a contrast medium region 56 appears as shown in FIG. 12A. Then, there is a demand to remove the contrast medium region 56 for rendering as shown in FIG. 12B. The removal can be done by performing segmentation (region extraction) of the contrast medium region 56.
To remove the contrast medium region 56 from the rendering object, hitherto a method of transforming CT values of the extracted region into values that will not be rendered (such as air) has been conducted. FIG. 13 is a graph to show the correspondence between the CT values of the extracted region before transform and those after transform in the image processing method in the related art. As shown in the figure, hitherto the CT values of air in a region, tissue, and contrast medium range have been transformed uniformly into the same value (CT value somewhere represents air). That is, CT values in the region determined to be the contrast medium region in the segmentation process is transformed into the uniform CT value.
FIG. 14 is a flowchart to show the image processing method in the related art. In the image processing method in the related art, the volume data of an organ is acquired from a CT apparatus (step S51) and, contrast medium region is extracted. For example, residual remained in the intestine which contains contrast medium shall form contrast medium region (step S52). The CT values of the extracted region are transformed into the CT value somewhere represents air (step S53), and the transformed CT values are used to silhouette the organ (step S54). A mask can be used instead of CT value transforming as an alternative rendering method.
Next, the terms concerning the internal tissue regions of a human body will be discussed with FIG. 15. Here, relative to a tissue 51 of the inside of a human body such as a large intestine and its neighbor, a region 53 is called “lumen,” a wall surface 54 is called “inner wall surface,” and a region 55 is called “inside of wall.” The lumen 53 is separated into an air region 52 and a contrast medium (residual) region 56.
FIG. 16 is a drawing to describe a problem of the image processing method in the related art. According to the image processing method in the related art, if an attempt is made to segment a contrast medium region and render a region 57 where the contrast medium region is removed by using the extracted region, upon rendering, a step is generated at the position of the interface between the air region 52 and the region 57 where the contrast medium region is removed as indicated by an arrow D, and a polyp, etc., existing in that portion cannot clearly be observed.
FIGS. 17A-17C show drawings for explaining profiles of change of CT values in the tissue, the air region and the contrast medium region. FIG. 17A shows, in rendering the organ containing a residual, the tissue 51 such as an intestine, the air region 52 of the lumen, and the contrast medium region 56 produced by a contrast medium remained in the lumen.
FIG. 17B shows the CT values and opacity values (derived from CT values) on a profile line p0-p2 passing through a point p0 in the air region 52 of the lumen, a point p1 in the inner wall surface of the tissue 51, and a point p2 in the tissue 51. FIG. 17C shows the CT values and opacity values on a profile line p3-p5 passing through a point p3 in the contrast medium region 56 of the lumen, a point p4 in the inner wall surface of the tissue 51, and a point p5 in the tissue 51. Usually, the opacity value is uniquely calculated using a lookup table (LUT) from the CT values.
The CT values and the opacity values on the profile line p0-p2 from the point p0 to the point p2 make a smooth transition between the air region 52 and the tissue 51 as shown in FIG. 17B. On the other hand, for the CT values and the opacity values on the profile line p3-p5 from the point p3 to the point p5, if the CT values of the contrast medium region 56 left in the tissue 51 are close to those of the tissue 51 as indicated by an arrow E in FIG. 17C, it becomes difficult to discriminate between the contrast medium region 56 and the tissue 51, and a virtual ray is blocked by a portion where an opacity value is high in the contrast medium region 56. Thus, in the image processing method in the related art, the quality of the volume rendering is impaired by the presence of the contrast medium region 56.
FIGS. 18A-18C are drawings to describe cause (1) of the problem in the image processing method in the related art. FIG. 18A shows, in rendering the organ containing a residual, the tissue 51 such as an intestine, the air region 52 of the lumen, and the region (extracted region) 57 where the contrast medium region is virtually removed.
FIG. 18B shows the CT values and opacity values on a profile line p0-p2 passing through a point p0 in the air region 52 of the lumen, a point p1 in the inner wall surface of the tissue 51, and a point p2 in the tissue 51. FIG. 18C shows the CT values and opacity values on a profile line p3-p5 passing through a point p3 in the extracted region 57, a point p4 in the inner wall surface of the tissue 51, and a point p5 in the tissue 51.
The CT values and the opacity values on the profile line p0-p2 from the point p0 to the point p2 make a smooth transition according to the resolution in the boundary portion between the air region 52 and the tissue 51 as indicated by an arrow F in FIG. 18B. On the other hand, the CT values and the opacity values on the profile line p3-p5 from the point p3 to the point p5 change discontinuously due to binary extraction as indicated by an arrow G in FIG. 18C, and thus a step is generated and smoothness is lost in the method of uniformly rendering the inside of the extracted region 57.
FIGS. 19A and 19B are drawings to describe cause (2) of the problem in the image processing method in the related art. The figures show correction of the extracted region. FIG. 19A shows the contrast medium region 56 before correction, and here a lesion part (polyp), etc., indicated by an arrow H exists. FIG. 19B shows the region 57 after correction, and the shape of the lesion part, etc., indicated by an arrow J is deformed. Thus, the shape of the lesion part, etc., also changes by correcting the extracted region, and therefore a precise diagnosis is hindered and it may become difficult to calculate the precise deformation amount. Thus, it is not desirable to solve the problem shown in FIGS. 18A-18C by correcting the extracted region. Even by this method, it is not possible to completely remove the discontinuous change caused by binary extraction of opacity value, and therefore this method does not lead to a substantial solution.